1. Field of the Invention
This invention relates to a variable capacitor capable of obtaining smooth and moderate capacity variation.
2. Description of the Prior Art
A PN junction diode is generally used as a variable capacity diode, because, when reverse bias voltage is applied to the PN junction, carriers near the junction move so as to form a depletion layer, and the thickness of the depletion layer varies with the reverse bias voltage.
One form of such a variable capacitor diode has such a structure wherein an electrode to which bias voltage is applied also serves as the electrode for reading the capacity variation. Therefore, if a bulk having a uniform doping is used, the capacity variation characteristic is such that capacity variation is relatively moderate when a high bias voltage is applied, thus causing necessity of controlling the diffusion profile by means of ion implantation and the like. This means difficulty in producing such devices and even if the control of diffusion profile can be performed, it is only useful over a narrow range. Further, due to the above-described structure, circuit design is somewhat restricted. Additionally, with supply voltage of about 12 V, which is a standard voltage in a car, etc, the thickness of the depletion layer formed in accordance with the movement of carriers is also restricted.